David Davis has threatened to resign on a number of occasions as Brexit Secretary.

There was no immediate comment from either Baker or Davis.

She said she had been told by a source that either Theresa May "dumps" the plan signed off at Chequers or "another minister will go, then another, then another, then another".

The prime minister faces hostile questioning in the Commons and a brutal meeting of the 1922 Committee later.

He said he was anxious the government's negotiating approach would "lead to further demands for concessions" from Brussels.

"It seems to me that the national interest requires a secretary of state in my department that is an enthusiastic believer in your approach, and not merely a reluctant conscript", he wrote.

The plan would see the United Kingdom sticking to European Union regulations in return for a free-trade deal with the bloc.

Conservative lawmaker Jacob Rees-Mogg, a leader of the party's "hard Brexit" faction, compared May's plan to an egg so softly boiled that it "isn't boiled at all".

Davis has been locked in bitter disputes with the Prime Minister and her chief Brexit adviser Olly Robbins for months.

Foreign Secretary Boris Johnson, a leading Brexit supporter, was widely reported to have described the plan as a "turd" before agreeing to support it.

The appointment comes after Davis resigned overnight, saying he would be unable to deliver the PM's post-Brexit trade strategy - which was approved by the cabinet on Friday - as he did not "believe" in it. But some senior pro-Brexit ministers have backed May's plan.